Optical waveguide arrays are used as routers and star couplers in optical communication systems. In integrated optics, a wavelength grating router can be realized efficiently by using a generalized Mach-Zehnder arrangement of many arms. One such wavelength grating router is described in my U.S. Pat. No. 5,136,671, issued on Aug. 4, 1992. As shown in FIG. 1, this router arrangement is generally symmetric and it is composed of two dielectric slabs, 101 and 102, two periodic arrays 103 and 104, and a set of waveguides (grating arms), 105, of different lengths between the two arrays. Typically, in commercial router array devices, the loss exceeds 5 dB for the central ports of the router and, for the marginal ports, it often exceeds 9 dB. Such losses seriously limit the usefulness of the router for passive networks and applications such as channel dropping filters with stringent requirements on loss uniformity. It is generally important to improve uniformity, by reducing the difference between maximum and minimum loss. The above loss variation is primarily caused by the waveguide grating. It is caused by scattering at the two junctions where the grating arms connect to the input and output slabs. Efficient router arrays are currently realized by means of adiabatic transitions that are generally difficult to realize, because they require very small gaps, 110, between adjacent waveguides. Theoretically, scattering at the two junctions can be reduced by reducing the gaps between adjacent cores in the vicinity of each junction. However, this approach generally results in higher crosstalk, because of nonuniform filling by the top cladding in the regions between the waveguide cores. Another approach, that is often used to improve efficiency, is to reduce the waveguides contrast, but this has the disadvantage of increasing the device dimensions, since it requires smaller curvatures in the bends of the arms.
Thus there is a continuing need to reduce the loss of such waveguide grating routers and star couplers.